The signal transduction and activator of transcription (Stat) gene family has been highly conserved throughout evolution. Gene duplication and divergence has produced 7 mammalian Stat genes, each of which mediates a distinct process. While some Stat proteins are activated by multiple cytokines, Stat2 is highly specific for responses to type I interferon. We have cloned mouse Stat2 and found that while its sequence was more divergent from its human homologue than any other mousehuman Stat pairs, it was fully functional even in human cells. Overall sequence identity was only 69%, compared with 85-99% similarity for other Stat genes, and several individual domains that still served similar or identical functions in both species were even less well conserved. The coiled-coil domain responsible for interaction with IRF9 was only 65% identical and yet mouse Stat2 interacted with either human or mouse IRF9; the carboxyl terminus was only 30% identical and yet both regions functioned as equal transactivation domains. Both mouse and human transactivation domains recruited the p300/CBP coactivator and were equally sensitive to inhibition by adenovirus E1A protein. Interestingly, the Stat3 carboxyl terminus also functioned as a transactivator capable of recruiting p300/CBP, as does the Stat1 protein, although with widely differing potencies. Yet these proteins share no sequence similarity with Stat2. These data demonstrate that highly diverged primary sequences can serve similar or identical functions, and that the minimal regions of similarity between human and mouse Stat2 may define the critical determinants for function.

Cell adhesion molecules of the Ig superfamily are implicated in a wide variety of biological processes, including cell migration, axon guidance and fasciculation, and growth control and tumorigenesis. Expression of these proteins can be highly dynamic and cell type specific, but little is known of the signals that regulate such specificity. Reported here is the molecular cloning and characterization of rat CDO, a novel cell surface glycoprotein of the Ig superfamily that contains five Ig-like repeats, followed by three fibronectin type III–like repeats in its extracellular region, and a 256-amino acid intracellular region that does not resemble other known proteins. In rat embryo fibroblasts, cdo mRNA expression is maximal in confluent, quiescent cells. It is rapidly and transiently down-regulated by serum stimulation of such cells, and is constitutively down-regulated in oncogene-transformed derivatives of these cells. CDO protein levels are also dramatically regulated by cell–substratum adhesion, via a mechanism that is independent of cdo mRNA expression. The amount of CDO produced at the surface of a cell may therefore be governed by a complex balance of signals, including mitogenic stimuli that regulate cdo mRNA levels, and substratum-derived signals that regulate CDO protein production. cdo mRNA is expressed at low levels in most adult rat tissues. A closely related human gene maps to chromosome 11q23–24, a region that displays frequent loss of heterozygosity in human lung, breast, and ovarian tumors. Taken together, these data suggest that loss of CDO function could play a role in oncogenesis.

Xenopus oocytes and eggs provide a dramatic example of how the consequences of p42 mitogen-activated protein kinase (p42 MAPK) activation depend on the particular context in which the activation occurs. In oocytes, the activation of Mos, MEK, and p42 MAPK is required for progesterone-induced Cdc2 activation, and activated forms of any of these proteins can bring about Cdc2 activation in the absence of progesterone. However, in fertilized eggs, activation of the Mos/MEK/p42 MAPK pathway has the opposite effect, inhibiting Cdc2 activation and causing a G2 phase delay or arrest. In the present study, we have investigated the mechanism and physiological significance of the p42 MAPK-induced G2 phase arrest, using Xenopus egg extracts as a model system. We found that Wee1-depleted extracts were unable to arrest in G2 phase in response to Mos, and adding back Wee1 to the extracts restored their ability to arrest. This finding formally places Wee1 downstream of Mos/MEK/p42 MAPK. Purified recombinant p42 MAPK was found to phosphorylate recombinant Wee1 in vitro at sites that are phosphorylated in extracts. Phosphorylation by p42 MAPK resulted in a modest ( approximately 2-fold) increase in the kinase activity of Wee1 toward Cdc2. Titration experiments in extracts demonstrated that a twofold increase in Wee1 activity is sufficient to cause the delay in mitotic entry seen in Mos-treated extracts. Finally, we present evidence that the negative regulation of Cdc2 by Mos/MEK/p42 MAPK contributes to the presence of an unusually long G2 phase in the first mitotic cell cycle. Prematurely inactivating p42 MAPK in egg extracts resulted in a corresponding hastening of the first mitosis. The negative effect of p42 MAPK on Cdc2 activation may help ensure that the first mitotic cell cycle is long enough to allow karyogamy to be accomplished successfully.

Rat 6 fibroblasts that overproduce protein kinase C 11 (R6-PKC3 cells) are hypersensitive to complete transformation by the T24 H-ras oncogene; yet T24 H-ras-transformed R6-PKC3 cells are killed when exposed to 12-O-tetradecanoylphorbol-13-acetate (TPA) Mol. Cell. Biol. 9:2641-2647, 1989. Treatment of an R6-PKC3 subclone that harbors a T24 H-ras gene under the control of an inducible mouse metallothionein I promoter with ZnSO4 and TPA is extremely cytocidal. This procedure was used to isolate rare revertants that are resistant to this toxicity. Two revertant lines, R-la and ER-1-2, continue to express very high levels of protein kinase C enzyme activity but, unlike the parental cells, do not grow in soft agar. Furthermore, these revertants are resistant to the induction of anchorage-independent growth by the v-src, v-H-ras, v-raf, and, in the case of the R-la line, v-fos oncogenes. Both revertant lines, however, retain the ability to undergo morphological alterations when either treated with TPA or infected with a v-H-ras virus, thus dissociating anchorage independence from morphological transformation. The revertant phenotype of both R-la and ER-1-2 cells is dominant over the transformed phenotype in somatic cell hybridizations. Interestingly, the revertant lines no longer induce the metallothionein I-T24 H-ras construct or the endogenous metallothionein I and II genes in response to three distinct agents: ZnSO4, TPA, and dexamethasone. The reduction in activity of metallothionein promoters seen in these revertants may reflect defects in signal transduction pathways that control the expression of genes mediating specific effects of protein kinase C and certain oncogenes in cell transformation.(Protein kinase C (PKC) plays a central role in signal transduction, mediating, at least in part, the effects of several oncogene products, growth factors, and cytokines and serving as the cellular receptor for the phorbol ester class of tumor promoters (5,53,54). Activation of PKC has pleiotropic effects, and it has been difficult to determine which effects are most intimately linked to growth control and tumor promotion. Furthermore, the events occurring between activation of PKC and induction of PKC-regulated genes are largely unknown. Dissection of the mechanisms by which PKC transduces signals and influences biological phenomena is complicated by the fact that PKC is encoded by a multigene family comprising at least eight members (4,5,53,54,57). One approach to this problem is the overproduction of single isoforms of PKC in various cell lines of interest (6,7,15,29,41,58,69